I'm trying to put a reference image next to my actual sketch. I see that Photoshop can open 2 windows at once side by side, but I'm not sure on how to put a referance image next to a sketch. Thanks!

You first need to set Krita's Multiple Document Mode to subwindows in preferences. (Then restart)
https://userbase.kde.org/Krita/Manual/P ... es/General

Then you can open multiple images and use "windows -> tile" to have images side by side.

example: transform mask tutorial - this shows using reference image side by side
https://krita.org/item/using-the-transform-masks/

There is also a "Reference" feature that works like the other smaller windows like,"Layers, Tools, Color wheel, etc." Go to the list of windows at the top bar and you should see it listed with the others. Simply drag it to the side and you will be able to browse images on your computer right next to your drawing.

Also If you are on Linux than you can right-click the image viewer you are using at the top bar and you should see "Always stay on top" as an option on the list. Change the window to its right size and then it will work pretty much the same way as the reference feature.

Lastly if you are on a Windows then there is a free program called "Pureref" which is designed for artists using reference.